The Baguio district contains a diverse array of epithermal, porphyry and skarn deposits, together with a large, broadly strata bound, advanced argillic lithocap. Magmatism, mineralization, and alteration occurred in response to subduction of the South China Sea plate and the Scarborough Ridge beneath northern Luzon over the past 3 m.y. Rapid uplift and exhumation resulted in epithermal veins overprinting several porphyry Cu-Au deposits. Most of the epithermal Au-Ag deposits of the Baguio district (including Antamok and Acupan, the two largest Au deposits) are intermediate sulfidation state quartz-carbonate-adularia-illite-base metal sulfide veins that contain electrum and minor Au-Ag tellurides. In contrast, high sulfidation mineralization at Kelly includes enargite, tennantite, electrum, and precious metal tellurides and is associated with advanced argillic alteration.
Although the mineralizing fluids that formed the porphyry and epithermal deposits had distinct temperatures and salinities, stable and radiogenic data provide evidence for direct magmatic contributions into each deposit type. The epithermal mineralizing fluids were dilute (generally, <2 wt % NaCl equiv) and had moderate temperatures (<300°C). Porphyry-style mineralization was associated with high temperature (300° to >600°C) hypersaline brines (30 to >70 wt % NaCl equiv) and low-density vapor. Sulfur isotope compositions of sulfides in the porphyry, skarn and intermediate sulfidation epithermal veins of the southern and central Baguio district are mostly between +1 and +6 per mil, consistent with a predominance of H
Intermediate sulfidation epithermal veins at Acupan have crosscut a well-mineralized porphyry Cu-Au stock-work at Ampucao. The two deposits cannot be distinguished on the basis of radiometric age determinations (Ampucao: 0.51 ± 0.26 Ma; Acupan: 0.65 ± 0.07 Ma), and are interpreted to be cogenetic, with telescoping of the two environments caused by the rapid uplift and exhumation associated with ridge subduction. Measured